Process gasses are delivered from a gas box to a process chamber for various applications, such as reactive ion etching applications for building transistors on silicon wafers. Process gases are mixed downstream of the mass flow controllers (MFCs) to a mixing manifold prior to delivery to the process chamber, such as a plasma reaction chamber. Therefore, it is necessary to achieve good mixing of very low flow and high flow rate carrier gases in the mixing manifold and deliver them without significant delays (within allowed gas settlement times) to the process chamber to perform the various applications, such as etching.
Transient gas flow delays to the process chamber, which are greater than the allowed gas settlement times, affect etch rates adversely for short process recipes (30 seconds to 60 seconds processes) due to non-stabilized or unsteady flows to the chamber. The problem is further enhanced due to hardware differences in various gas boxes causing different transport delays to process chambers to create etch rate matching issues. In a gas-box with multiple gas feeds of low and high flow rate gases, spatially separated in a random gas order from various MFCs, are bound to be delivered at different times to the process chamber depending upon their diffusivity and flow velocities (momentum or inertia).
The gas delay delivery problems may be attributed to the volume through which low flow gases flow to mix with the higher flow carrier gas(es). Delayed delivery of key process etching gases to the reaction chamber impacts wafer etch rates and critical dimensions on silicon wafers. In a mixing manifold with an isolated low flow gas, located away from a high flow gas, will take some physical length of time to mix with higher flow gas used to speed-up the deliver of the gas mixture to the chamber. The time required to fill the low flow gas volume from the MFC until it mixes with the high flow gas, as well as its diffusion through the high flow gas, determines the total transport delay to the reaction chamber.